沉淀法含砂仪测量时，用量筒下部取满钻井液是（）mL。
A、20
B、30
C、50
D、45
参考答案：C
The old man has retired from the factory. He _____ there for 30 years.
A．works
B．worked
C．has worked
D．was working
何谓CPAP通气模式？（中文名称）
